This invention relates to elongated coverings made of resinous plastics that are resistant to temperature stresses and/or aggressive media.
As new fields of application continue to be found for products already on the market, the characteristics of these products must be adapted to meet market requirements. Thus, to an increasing extent, materials that are resistant to high temperatures, generally based on fluoropolymers, are being used to insulate or cover electrical cable and wiring, e.g. in what is known as heating cable, as well as in the field of sensor technology, where temperatures in excess of 300.degree. C. must be reckoned with. In the automobile industry, for example, this applies to lead wiring for lambda probes, which, in combination with regulated catalytic converters, ensure that exhaust emission values are maintained at a low levels.
Other applications, such as taking samples in the chemicals industry, use small, media-conducting pipes that are exposed to elevated temperatures, but it is especially important that their functionality not be affected by ambient aggressive media. Therefore, for all these applications, it has long been known that fluoropolymers, which cannot be worked from a melt, may be used as insulation/covering for electrical cable and wiring, and that tube walls can be manufactured using such materials. It has proven particularly advantageous in this regard to use the fluoropolymer in strip form for the purposes specified above; the material of the strip may be sintered or unsintered (Swiss patent CH-PS 562 098; German patent DE-PS 32 14 447).
However, there are occasions, as in sensor technology, when it is important to have access to comparative values from the outside, at a measuring point, in order to regulate an operating system after comparing desired and actual values. This is the case, for example, with the lambda probe mentioned above, with which, essentially, a suitably responsive electrode extends into a flow of exhaust gas exiting from an engine block; one end of the electrode being exposed to the flow of exhaust gas, while the other end is in contact with outside air. This type of regulating system must be protected from dirt accumulation and splashed water, or one disadvantage is that it becomes increasingly difficult to make sufficient oxygen, for comparison, available to the regulating system from ambient air. In some cases, efforts to use open gusset space of the electrical conductors in the supply leads to this end have resulted in an introduction of splashed water, which has had a negative impact on the regulating process.
If an elongated tubular covering itself is used as a measurement probe or sensor, such as for monitoring electrical equipment, for pipelines and similar devices, e.g. to detect liquids or gases seeping from or penetrating into the system, and to report locations of entry or exit to a central monitoring station, an important thing is often to select a medium to perform this monitoring function so that false alerts are avoided.